I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for controlling transmission energy in a communications system employing orthogonal transmit diversity.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
CDMA by its inherent nature of being a wideband signal offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a remote user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390, entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
In other modulation schemes such as TDMA, signal diversity acts as noise to the receiver, and as such is highly undesirable. The value of diversity reception in CDMA systems, on the other hand, is so pronounced that systems have been developed to intentionally introduce signal diversity into the transmissions. One method of deliberately introducing signal diversity in a CDMA communication system is to transmit identical signals through separate antennas as described in U.S. Pat. No. 5,280,472, entitled xe2x80x9cCDMA Microcellular Telephone System and Distributed Antenna Systemxe2x80x9d, which is assigned to the assignee of the present invention and incorporated by reference herein.
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled xe2x80x9cThe cdma2000 ITU-R RTT Candidate Submissionxe2x80x9d. A second of these proposals was issued by the European Telecommunications Standards Institute (ETSI), entitled xe2x80x9cThe ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submissionxe2x80x9d.
The Telecommunications Industry Association has developed the initial cdma2000 submission into a draft specification entitled xe2x80x9cProposed Ballot Text for cdma2000 Physical Layerxe2x80x9d, hereafter referred to as the cdma2000. This draft specification describes a method of providing path and code space diversity referred to as Orthogonal Transmit Diversity (OTD). In OTD, the information to be transmitted to a remote station is demultiplexed into two signals. Each of the two signals is spread using distinct orthogonal spreading sequences and transmitted from different antennas.
A useful method of power control of a remote station in a communication system is to monitor the power of the received signal from the remote station at a base station. The base station in response to the monitored power level transmits power control bits to the remote station at regular intervals. A method and apparatus for controlling transmission power in this fashion is disclosed in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
Orthogonal spreading sequences are highly desirable in CDMA communications systems because the cross correlation between any two orthogonal sequences is zero. However, orthogonal sequences have very poor auto correlation properties and in mobile environments that encounter multipath effects the poor auto correlation properties would render a CDMA system inoperable. Because of this effect, a pseudonoise covering that covers the orthogonally spread data is highly desirable. The pseudonoise coverings are selected such that the correlation between the pseudonoise sequence and a time-shifted version of the sequence is low. In new high capacity systems, a method of spreading data so as to evenly distribute the loading on the in-phase and quadrature channels, referred to as complex PN spreading, has been developed. A method and apparatus for performing complex PN spreading is described in detail in copending U.S. patent. application Ser. No. 08/886,604, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
The present invention is a novel and improved method and apparatus for controlling transmission energy. The present invention describes a closed loop power control system that operates in conjunction with a transmitter using orthogonal transmit diversity. In a first embodiment of the present invention, the receiver evaluates the signal to noise ratio (SNR) of the two OTD components of the signal. A weighted sum of these two components emphasizing the weaker of the two signals is generated and used in the generation of the power control commands. In a second embodiment of the present invention, the SNR of the two component signals are calculated and two separate power control commands are generated based on the corresponding calculated SNR values.